ice: make writes to /dev/gnssX synchronous

commit bf15bb38 upstream.

The current ice driver's GNSS write implementation buffers writes and
works through them asynchronously in a kthread. That's bad because:
 - The GNSS write_raw operation is supposed to be synchronous[1][2].
 - There is no upper bound on the number of pending writes.
   Userspace can submit writes much faster than the driver can process,
   consuming unlimited amounts of kernel memory.

A patch that's currently on review[3] ("[v3,net] ice: Write all GNSS
buffers instead of first one") would add one more problem:
 - The possibility of waiting for a very long time to flush the write
   work when doing rmmod, softlockups.

To fix these issues, simplify the implementation: Drop the buffering,
the write_work, and make the writes synchronous.

I tested this with gpsd and ubxtool.

[1] https://events19.linuxfoundation.org/wp-content/uploads/2017/12/The-GNSS-Subsystem-Johan-Hovold-Hovold-Consulting-AB.pdf
    "User interface" slide.
[2] A comment in drivers/gnss/core.c:gnss_write():
        /* Ignoring O_NONBLOCK, write_raw() is synchronous. */
[3] https://patchwork.ozlabs.org/project/intel-wired-lan/patch/20230217120541.16745-1-karol.kolacinski@intel.com/



Fixes: d6b98c8d ("ice: add write functionality for GNSS TTY")
Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Tested-by: Sunitha Mekala <sunithax.d.mekala@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Liwei Song <liwei.song@windriver.com>